This invention relates to a method of treating a feed gas stream comprising hydrogen sulphide.
Gas streams comprising hydrogen sulphide are formed for example as waste gases in an oil refinery or gas refinery operation. In view of their hydrogen sulphide content, these gas streams cannot be discharged to the atmosphere without first being treated so as to remove almost all the hydrogen sulphide.
A standard method of treating such a gas stream serves to recover sulphur therefrom is by the Claus Process. Conventional Claus processes are described in the introductory paragraphs of EP-A-565 316.
EP-A-565 316 discloses a process in which in a first reactor a part of the hydrogen sulphide content of a feed stream comprising hydrogen sulphide is oxidised to sulphur dioxide, and so formed sulphur dioxide is reacted with residual hydrogen sulphide to form sulphur vapour and water vapour. A partially reacted gas stream including sulphur vapour, water vapour, residual hydrogen sulphide and residual sulphur dioxide is withdrawn from the furnace. A sulphur condenser is employed to extract sulphur vapour from the partially treated gas stream so as to form a sulphur vapour depleted gas stream. At least part of the sulphur depleted gas stream is sent to a further reactor in which its sulphur dioxide content is reduced to hydrogen sulphide. Water vapour is extracted from the resulting reducer gas stream. The water vapour-depleted gas stream is then preferably recycled to the furnace. A purge stream is taken from a chosen position in the above described cycle and is subjected to further treatment so as to render it fit for discharge to the environment. The purpose of the recycle is to obtain a very high effective conversion of hydrogen sulphide to sulphur vapour in the furnace and thereby facilitate the attainment of a total conversion efficiency which is sufficient to meet any prevailing environmental standard.
By using pure oxygen (or air highly enriched in oxygen) the size of the initial furnace may be kept down. However, the advantages in size reduction of the initial purge gas made possible by the use of pure oxygen (or oxygen highly enriched in air) as the oxidant are counteracted by the recycle of gas to the furnace. Although EP-A-0 565 316 further discloses that the recycle can be omitted, this is stated not to be preferred as it has an adverse effect on the effective percentage conversion of hydrogen sulphide to sulphur in the furnace.
One solution to this problem suggested in EP-A-565 316 is to employ an amine separation unit to concentrate the recycle stream in hydrogen sulphide. Such amine separation units, however, tend to be particularly costly, even if only of a small size.
It is an aim of the method according to the invention to provide an alternative solution to this problem which does not necessitate a recycle.